In general, all the lines show that there is an increasing trend of people with bronchitis.
When students are preparing for IELTS Task 1 writing they learn the importance of describing ‘trends’ in graphs, tables and charts. Not surprisingly, they go ahead and use the word ‘trend’ to describe these trends. But native speakers almost never do that.
In the opening example a trend is described, but the word ‘trend’ is redundant. It is enough to write:
This kind of statement stands out as ‘a trend’ because it says something general about a change over time without mentioning data.
So here’s your checklist for a trend:
Understand also, that a trend is often ‘hidden’ in data that is highly irregular. In the following graph grammatical accuracy goes up and down erratically over time, but the general trend (shown by the straight line) is downward.
The ‘trend’ in this graph can be described:
The more frequently the word ‘trend’ is used, the less accurate the writing.